New Insights into Biobased epoxy resins: synthesis and characterization

The synthesis and characterization of a variety of novel biobased epoxy thermosets comprising plant oil-derived epoxy prepolymers and dicarboxylic acids (DCAs) and/or glutaric anhydride, in the presence of amine curing agents, in order to develop new insights on structure-property relationships, are reported.
The effect of:
i. different epoxy prepolymers: epoxidized linseed oil (ELO); epoxidized soybean oil (ESBO), and; their corresponding epoxidized methyl esters EML and EMS;
ii. systematically increasing carbon-carbon chain length of a series of α,ω-dicarboxylic acids (DCAs);
iii. different curing conditions based on type and concentration of accelerant: N,N-4-dimethylaminopyridine (DMAP); 1-methylimidazole (1-MI); 2-methylimidazole (2-MI); vinylimidazole (VI), and; trimethylamine (TEA), cure temperature and time.
iv. mixtures glutaric anhydride-adipic acid, and;
v. inclusion of native and modified starch to ELO-adipic acid resins,
on thermal and mechanical properties are discussed in detail.
Resins derived from DCAs gave soft and flexible materials. ELO-derived samples possessed higher Tg and better mechanical properties due to their higher oxirane content and functionality. Systematically increasing the carbon-carbon chain length improves thermal stability but reduces Tg, tensile strength, Young’s modulus, elongation at break and toughness. Among the various accelerants trialed, DMAP gave samples with best properties while TEA gave samples with poorest properties. Increasing DMAP concentration resulted in improved Tg, tensile strength and Young’s modulus but reduced thermal stability.
Resins derived from glutaric anhydride gave hard and rigid materials. By changing the ratio of glutaric anhydride to adipic acid, resins with various properties ranging from soft flexible to hard tough materials were attained.
Biobased thermoset composites comprising ELO-adipic acid and starch (native and modified) accelerated by DMAP, showed increased Young’s modulus while decreased tensile strength and elongation at break.